This invention relates to ceramic dielectric coating compositions, processes for depositing the coating compositions on a substrate, and the coated articles resulting therefrom. More particularly, this invention relates to a ferroelectric or piezoelectric coating composition containing a phosphate additive, a process for applying the composition to a substrate and the resulting composite article of the substrate with an adherent coating composition thereon.
The ferroelectric effect is the property of certain crystalline dielectric materials when polarized to retain their polarization. The piezoelectric effect is the property of certain crystalline dielectric materials when polarized to generate an electric field or potential of one polarity when a compressive force is applied and the polarity is reversed when a tensile or stretching force is applied. This effect is reciprocal in that the material will tend to compress if an electric field or potential of one polarity is applied across the material and the material will tend to expand if the polarity is reversed.
Piezoelectric materials were initially limited to crystals such as quartz and rochelle salt which were grown under controlled conditions as single crystals and were sliced into functional pieces with selected orientations with respect to crystal axes. More recently, piezoelectric materials which include a variety of polycrystalline ceramics can be processed into functional parts in a more economical manner by using techniques common to ordinary ceramic products. The parts are formed "green" and must be fired at high temperatures (about 1300.degree. C.) to fuse the microscopic granules and produce a hard ceramic which can exhibit the piezoelectric effect after the application of a polarizing electric field. The fired parts may then be sliced or shaped with abrasive processes such as grinding or polishing usually being required. As can be appreciated, any means which obviates or minimizes costly slicing and abrading procedures would represent a contribution to this art.
One approach to obviate or minimize such procedures has been to produce piezoelectric films or coatings. An organic type binder vehicle can be employed with a powdered piezoelectric material but the vehicle either tends to distort the film or coating as it volatilizes during sintering of the material or else is so viscous as to prevent the formation of a smooth film or coating. Alternatively, the material can be powdered and milled with distilled water and then sprayed onto a substrate, dried and sintered with layers being successively built up in this manner. These coating approaches have limitations or disadvantages, such as, the coatings require sintering after application which utilizes high temperatures and oftentimes results in deficiencies in the coating and the coatings require some type of bonding to a substrate after formation which utilizes an added and costly procedure and oftentimes results in deficiencies in the bond. As can be appreciated, a coating which is dried or cured at relatively low temperatures, can be applied over various areas and shapes, as desired, and strongly adheres to or is formed integral with the substrate would represent an advancement in or contribution to this art.